Impulse radar system utilizing pulse duration modulation

ABSTRACT

1. An impulse radar system operating with high pulse repetition frequency and doppler frequency evaluation and employing a plurality of distance measuring branches which are, for the measurement of the pulse frequency, by means of gate switches successively scanned by sampling pulses, comprising means at the receiver side for pulse duration modulating the pulses transmitted, at the receiver side for demodulating pulse duration modulation applied periodically at the transmitting side so as to produce rough distance measuring pulses, a coincidence circuit, and means for conducting said rough distance measuring impulses together with other pulses to said coincidence circuit, operative to form fine pulses upon coincidence of the pulses conducted thereto, which permit unequivocal distance indication even in the presence of high pulse repetition frequency.

United States Patent 1191 [111 3,745,576

Baur et a1. 1 July 1%, 1973 1 IMPULSE RADAR SYSTEM UTILIZING EXEMPLARYCLAIM PULSE DURATION MODULATION 1. An impulse radar system operatingwith high pulse [75] Inventors: Friedrich Baur, Muenich-Maxhof;repetition frequency and doppler frequency evaluation Jl'irgen Naujoks,Munich, both of and employing a plurality of distance measuring Germanybranches which are, for the measurement of the pulse frequency, by meansof gate switches successively scanned by sampling pulses, comprisingmeans at the receiver side for pulse duration modulating the pulses [22]Filed: June 1, 1961 transmitted, at the receiver side for demodulatingpulse duration modulation applied periodically at the trans- [2]] Appl'114266 mitting side so as to produce rough distance measuring pulses, acoincidence circuit, and means for conducting [73] Assignee: SiemensAktiengesellschatt, Berlin and Munich, Germany [52] 11.5. C1. 343/13 R,343/ 17.2 R said rough distance measuring impulses together with [51]Int. Cl. G015 9/06 other pulses to said coincidence circuit, operativeto [58] Field of Search 343/14, 17.2, 13 form fine pulses uponcoincidence of the pulses conducted thereto, which permit unequivocaldistance in- [56] References Cited dication even in the presence of highpulse repetition UNITED STATES PATENTS frequency- 2,556,l09 6/1951 Rustet al 343/14 2,753,448 7/1956 Rines 343/172 5 C1 'F PrimaryExaminer-r-Malcolm F. Hubler 1 Drawing lgure Attorney-Hill, Sherman,Meroni, Gross & Simpson INTERMEDIATE TRAM$M117ER MdDL/ A T01?MULI/CLJ/FR (FREQ UEN CY PULSE DURA TION --v Mani/LA TO i Ml/L TIPL/EKIMPULSE RADAR SYSTEM UTILIZING PULSE DURATION MODULATION This inventionis concerned with an pulse radar system operating with high pulserepetition frequency and doppler frequency evaluation.

The doppler frequency shift occurring in connection with moving objectsis utilized in the pulse doppler frequency radar system for the purposeof distinguishing between fixed and moving targets. Since the pulsecharacter and therewith the distance information are lost in the case ofsimple filters, it has been proposed to use for the pulse timingmeasurement separate distance measuring branches which are by means ofgate switches successively scanned with sampling pulses. Identicaldoppler frequency filters can in such case be inserted in the individualdistance measuring branches.

Difficulties arise in the doppler frequency system owing to the socalledinterfering frequencies which occur when the doppler frequency is equalto or a multiple of the impulse repetition frequency of the radarapparatus. In order to avoid detrimental effects on the measurement, thedata of the apparatus must be selected so that the first interferingfrequency lies at the upper limit of the measuring range, if possibleabove the maximum radial speed of the object concerned with themeasurement. The interfering frequency can be increased by increase ofthe carrier wave length or by increase of the pulse repetitionfrequency.

Increase of the wave length results in antennae structures withexcessively large dimensions and in other drawbacks connected therewith,and it is, therefore, more advantageous to effect an increase of thepulse repetition frequency while utilizing a fixed predetermined carrierwave length which is as short as possible.

However, the pulse repetition frequency cannot be readily increased inconnection with apparatus of considerable transmitter output andrelatively great range because echos from previous pulses would returnafter each transmitted pulse. Such echos result in false distanceindication if the deflection frequency of the radar sighting tube is, asis customary, equal to the pulse repetition frequency. If the deflectionof the indicating tube is derived by division from the pulse repetitionfrequency, there will result a multiple indication. However, suchdivision must be frequently employed in order to extend the indicatingrange beyond the range that may be calculated from the pulse repetitionfrequency. Accordingly, an impulse repetition frequency which is inconsideration of the first interfering frequency strongly increased,leads to a false or to an ambiguous indication and therewith touncertain measuring results. The object of the present invention is toovercome this drawback.

The German Pat. No. 911,663 describes an pulse doppler frequency radarsystem employing a periodic frequency variation at an impulse modulatedtransmitter with evaluation of the frequency shift of the receivedfrequency with respect to the transmitted frequency. The distance isthereby determined by consideration of the pulse running time as well asthe frequency shift. This is effected by utilizing the pulse runningtime for rough measurement while employing the signals from fixedtargets while indicating moving targets if both are at the samedistance.

Copending U.S. application Ser. No. 88,580, filed Feb. 10, 1961, ownedby the assignee also noted in the present case, proposes to avoid thedifficulties arising in connection with an pulse doppler frequency radarsystem due to interfering frequencies, by an increase of the pulserepetition frequency. It is proposed to employ an auxiliary periodicfrequency modulation at the transmitter so as to obtain from thefrequency shift of the received frequency, with respect to thetransmitted frequency, rough distance pulses which produce unequivocalindication of the fine distance measurement.

Another copending U.S. application Ser. No. 88,581, filed Feb. 10, 1961,likewise owned by the assignee named in the present case, proposes toemploy for the same purpose an auxiliary amplitude modulation of thepulse modulated transmitter carrier wave.

It will be seen, therefore, that the principle involved in bothproposals according to the two above noted prior applications, residesin selecting the distance pulses which are very accurate but lead to afalse or equivocal indication, by a second distance information whichneed not be accurate but which must be unequivocal. As a consequence,there are obtained highly accurate distance measuring pulses whichinvariably result in correct and unequivocal measurements. The advantageresides in the possibility of considerably increasing the pulserepetition frequency without detrimentally affecting the measurementaccuracy and obtaining measurement free of ambiguity. The high pulserepetition frequency permits placing of the interfering frequency veryhigh and to produce occurrence of further interfering frequenciesoutside of the range of the speed of aircraft which is of interest.

The proposals according to the copending applications, concerning theuse of-auxiliary frequency modulation and amplitude modulation,respectively, so as to obtain unequivocal indication, do not constitutethe only possibilities for the solution of the problems involved, vnordo they constitute in some circumstances the best possible solutions sofar as the expenditure and the interference spacing are concerned.

The present invention proposes a further solution which presentsadvantages, such solution contemplating the use of periodic, preferablysine shaped pulse duration modulation of the transmitted pulses so as toobtain at the receiver, by demodulation of the pulse durationmodulation, rough distance measurement pulses which are in a coincidencecircuit combined with the previously noted equivocal fine measuringpulses, thereby forming upon coincidence of rough measurement pulseswith fine measurement pulses accurate measuring pulses which permitunequivocal distance indication even in the presence of high pulserepetition frequency.

The various objects and features of the invention will appear from thedescription which is rendered below with reference to the accompanyingdrawing showing an example of a circuit of a radar system adapted forrealizing the invention.

The system comprises a transmitter T and a receiver R whichadvantageously operate, for good decoupling, respectively with separaterotating antennas TA and RA. Character 0 indicates a quartz stabilizedoscillator from which is obtained, by frequency multiplication in amultiplier stage V1, a high carrier frequency in the centimeter ormillimeter wave range, which is conducted to the transmitter stage Tover a modulator M0. Reference DSl indicates a divider stage forobtaining from the oscillator O, in the impulse stage 15, the pulsesequence frequency fi which is used for modulation in the transmitter T.However, before the pulses from the impulse stage IS reach the modulatorMo, they pass through a pulse duration modulator PDM in which they areas to duration thereof modulated with a frequency fm. The modulationfrequency fm for the pulse duration modulation is likewise derived fromthe oscillator over a further divider stage DS2, but is in considerationof the maximum measured frequency spacing lower than the pulserepetition frequency. The receiver stage R is connected with a mixingstage Mix, in which is effected heterodyning with a frequency whichdiffers by the amount of the receiver intermediate frequency from thetransmitter carrier frequency. This frequency can be derived from theoscillator 0 over a further multiplier stage V2.

Moving and stationary targets can be differentiated in known manner bythe use of doppler frequency filters. As is likewise known, comb filtersare suitable for this purpose as are, in given cases, simple filters.The demodulated received frequency is in the illustrated circuitconducted from the intermediate frequency stage IF to a comb filter CF,which filters out the frequencies not containing doppler frequencieswithout detrimentally affecting the signal. The comb filter is providedto prevent simulation of moving targets by the noise products appearingsubsequently upon possible subdivision of the fixed target echos in thegate circuits.

The comb filter CF is connected with a plurality of distance measuringcircuits, five in the present case, which are successively brieflyswitched in by means of gate switches GSl G55, that is, they are scannedwith socalled sampling pulses. An electronic timing circuit TC is, forexample, employed for controlling the gate circuits, such timing circuitbeing controlled from the impulse stage IS, with the pulse repetitionfrequency of the apparatus, placing continuously successively accuratesampling pulses on the gate circuits G51 G55, so that any succeedinggate is closed when the preceding gate is being opened. As soon as anpulse is received within the accurately defined interval in which one ofthe gate switches is closed, it will enter the corresponding distancemeasurement branch. The signals arriving from moving targets arethereupon conducted to the pulse demodulators PDI PD5 so as to obtainagain the pulse duration modulation frequency fm prevailing at thetransmitter side. The modulation frequency thereafter passes through aband pass filter Bl B5 which is tuned thereto. The phase position ofthemodulation frequency obtained at this point depends upon the distance ofthe target. It is accordingly possible to produce upon zero passage adistance pulse which indicates by its position as to time the targetdistance more or less roughly. This impulse production is effected in apulse shaper circuit such as J1 J5. A circuit may be employed for thispurpose which releases a sawtooth voltage or the like, upon zero passageor upon passage through a predetermined voltage value. Such a circuit isfor example known under the name of multiar circuit.

The relatively rough but unequivocal distance measurement pulse obtainedfrom the pulse duration modulation, is conducted to one input of acoincidence circuit such as C1 C5, to the other input of which areconducted the fine measurement pulses proper from the timing measurementof the reflected pulses. in the present case, the echo pulses whichappear at the outputs of the respective gates GSl G55, and which are inview of the high pulse repetition frequency, in effect, intermixed, areconducted to the respective coincidence circuits as fine pulsesv Thecoincidence circuit involved delivers an output signal only uponcoincidence of a rough distance measuring pulse with an accurate finemeasuring pulse, thereby extracting the latter.

There will accordingly appear at the outputs of the coincidence circuitsexact and accurate distance measuring pulses which are conducted to thecathode ray tube CR disposed in the radar sighting device for the brightscanning of the beam. The electron beam of the sighting tube is radiallydeflected by the action of a deflection coil DC controlled by adeflection generator DG, such deflection coil rotating synchronouslywith the radar antenna while the deflection device operatessynchronously with the modulation frequency fm. In order to effect therequired synchronization, the deflection device DG is connected with apoint of the transmitter circuit at which the modulation frequencyappears, for example, at the output of the divider stage DS2 or theinput of the pulse duration modulator PDM.

When selecting a very short wave length, in the centimeteror microwaverange, the system according to the invention will permit the use ofpulse repetition frequencies of the radar apparatus in the order ofmagnitude of 25 kilocycles. There may be used a considerably lower pulseduration modulation frequency frn of a few cycles. in this example,there will result a doppler frequency shift of 25 kilocycles at a speedof the target of about 1,350 kilometers per hour for a carrier wavelength of about 3 centimeters. The first interfering frequency occurs inthis example at a speed of 1,350 kilometers per hour.

Changes and modifications may be made within the scope and spirit of theappended claims which define what is beiieved to be new and desired tohave protected by Letters Patent.

We claim:

1. An impulse radar system operating with high pulse repetitionfrequency and doppler frequency evaluation and employing a plurality ofdistance measuring branches which are, for the measurement of the pulsefrequency, by means of gate switches successively scanned by samplingpulses, comprising means at the receiver side for pulse durationmodulating the pulses transmitted, means at the receiver side fordemodulating pulse duration modulation applied periodically at thetransmitting side so as to produce rough distance measuring pulses, acoincidence circuit, and means for conducting said rough distancemeasuring impulses together with other pulses to said coincidencecircuit, operative to form fine pulses upon coincidence of the pulsesconducted thereto, which permit unequivocal distance indication even inthe presence of high pulse repetition frequency.

2. A system according to claim 1, wherein said other pulses are echopulses.

3. A system according to claim 1, wherein said other pulses are samplingpulses.

3,745 ,576 6 4. A system according to claim 1, wherein said other pulseduration modulation of the transmitted pulse. and pulses are timingpulses which control the release of sampling pulses.

5. A system according to claim 1, wherein in said pulses so modulatedmodulating means is constructed to produce sine wave 5 said demodulatingmeans is constructed to demodulate

1. An impulse radar system operating with high pulse repetitionfrequency and doppler frequency evaluation and employing a plurality ofdistance measuring branches which are, for the measurement of the pulsefrequency, by means of gate switches successively scanned by samplingpulses, comprising means at the receiver side for pulse durationmodulating the pulses transmitted, means at the receiver side fordemodulating pulse duration modulation applied periodically at thetransmitting side so as to produce rough distance measuring pulses, acoincidence circuit, and means for conducting said rough distancemeasuring impulses together with other pulses to said coincidencecircuit, operative to form fine pulses upon coincidence of the pulsesconducted thereto, which permit unequivocal distance indication even inthe presence of high pulse repetition frequency.
 2. A system accordingto claim 1, wherein said other pulses are echo pulses.
 3. A systemaccording to claim 1, wherein said other pulses are sampling pulses. 4.A system according to claim 1, wherein said other pulses are timingpulses which control the release of sampling pulses.
 5. A systemaccording to claim 1, wherein in said modulating means is constructed toproduce sine wave pulse duration modulation of the transmitted pulse,and said demodulating means is constructed to demodulate pulses somodulated.